Added live-mode chunk calculation to ODIDDataSource using average content bitrate

    Live streaming of B-MP2PS_N-1 revealed some interoperability issues with
     low-bitrate content. e.g. the default chunk size of 1572864 (1536K) contains
     more than 1 second's worth of data on multiple occasions when streaming
     this content.

    Remedy:

     The MediaEngine no longer maintains the policy for the default chunk size.
     If it's not set in rygel.conf, "0" is passed to the DataSource which
     establishes the default value.

     For static data sources, the MediaEngine will use the default chunk size
     when "0" is passed to the constructor. For live sources, the chunk size
     is now set to 1/2 the average bytes-per-second for the source content.

     For live sources, added a LIVE_CHUNKS_PER_SECOND value that's used to
     calculate the chunk size to use on live streams based on the average
     bitrate of the content. The smaller of this value or DEFAULT_CHUNK_SIZE
     will be used as the effective chunk size for live streaming.

    Since there was yet another place where content size was being utilized,
     added ODIDUtil.file_size().

    Tested live streaming with B-MP2PS_N-1. Playback was smooth with the reduced
     chunk size.

Note: The rygel.conf chunk-size will currently override the default static
      chunk size *and* the dynamic live mode chunk size. So it should not
      be set in rygel.conf except when there's a need to override the
      default value/behavior.

unless there's a need to
      override the default static